Information is stored using a multiplicity of different storage media. One of these storage media is ferroelectric materials, which are characterized by spontaneous polarization. The polarization can be changed by applying an electrical field. This is utilized in the case of ferroelectric storage capacitors, for example. Depending on whether the polarization points in one direction or in the opposite direction, the storage capacitor stores a logic 1 or a logic 0.
Ferroelectric storage capacitors are the subject of intensive development. A prerequisite for the use of ferroelectric storage capacitors is that the ferroelectric layer, including the capacitor electrodes which adjoin this layer, needs to be patterned, however. The storage density which can be achieved using storage capacitors is determined essentially by the smallest structure size F which can be achieved using lithography. Storing one bit normally requires an area of at least 4 F2. On a commercial scale, F is currently approximately 120 nm. The storage density which can be achieved as a result is approximately 1.7 Gbit/cm2. On a laboratory scale, on the other hand, “focused ion beam milling” has already been able to be used to achieve a structure size of 70 nm (C. S. Ganpule, “Scaling of ferroelectric and piezoelectric properties in Pt/SrBi2Ta2O9/Pt thin films”, Appl. Phys. Lett. 75 (1999), 3874-3876). A significantly higher storage density is desired, however.
To this end, a large number of examinations were started to store information in a ferroelectric material in another way. Thus, U.S. Pat. No. 6,064,587, for example, proposes the use of a conductive tip to which an electrical potential is applied and which is passed over the surface of a ferroelectric material and modifies the polarization of individual domains of the ferroelectric material. To read the stored information, methods based on various physical principles are proposed. In one of these methods, the deflection of a nonconductive piezoelectric tip which has been placed close to the individual domains is evaluated. A further option proposed is the utilization of the electrooptical effect, which involves the change in the polarization of light being recorded by means of a near-field optical system. Finally, the use of nonlinear optical effects (for example Kerr effect, second harmonic generation and two or four wave mixing) is also proposed.
In U.S. Pat. No. 5,835,477, the polarization in the ferroelectric material is changed using an AFM tip which has been placed close to the ferroelectric material and to which a voltage is applied, and the information written in this manner is read by measuring a tunnel current between the AFM tip and the ferroelectric material.
Electrical writing using an AFM tip is also described by C. H. Ahn et al., “Local, Nonvolatile Electronic Writing of Epitaxial Pb(Zr0.52Ti0.48)O3/SrRuO3 heterostructures”, Science, Vol. 276 (1997), 1100-1103.
A drawback of this practice is that during reading an electrical field acts on the ferroelectric material and this results in the risk of alteration of the stored information in this domain and adjoining domains. Certain “nondestructive reading” is not assured.
When the information is written into the ferroelectric material using an electrical field, there is likewise the risk that scattering of the electrical field will likewise change the polarization of neighboring domains. Individual domains therefore cannot be actuated with sufficient isolation.
A further basic option for storing information using ferroelectric layers may be seen in the use of ferroelectric field effect transistors (S. Mathews et al., “Ferroelectric Field Effect Transistor Based on Epitaxial Perovskite Heterostructures”, Science, 276 (1997), 238-240).